The present invention relates to rotary piston internal combustion engines.
More particularly, but not exclusively, the invention relates to a so called Wankel engine in which a rotary piston, or so called rotor, rotates within a cavity formed by a rotor housing, the rotor outer periphery and the inner peripheral surface of the cavity being so shaped that working chambers are formed between the rotor and the inner peripheral surface which vary in volume as the rotor rotates, the cavity being provided with inlet and exhaust ports. In the best known example of an engine of the kind referred to, the cavity includes a stationary rotor housing having a two lobed epitrochoidal shaped cavity and a rotor of substantially triangular shape but with convex arcuate flanks, seals or so called apex seals at the apices of the rotor maintaining sealing contact with the inner peripheral surface of the cavity of the rotor housing, and seals or so called side seals, at the sides of the rotor maintaining sealing contact with two axially spaced end casings, and the rotor rotating in a planetary manner within the cavity.
One known method of cooling the rotor of an engine of the kind referred to is to circulate oil though internal passages in the rotor. Disadvantages of this system include considerable design complexity to seal the oil within the rotating and orbiting rotor, the space required for the oil seals prohibiting use of engine geometry of the more compact kind; and some power and thermal efficiency is lost through friction of the oil seals combined with the so called cocktail-shaker losses of the oil inside the rotor.
A known alternative method of cooling the rotor is to use air as the coolant. This has advantages of simple construction, more compact engine geometry, and low mechanical friction losses.
In one version, one or more passageways are formed in the rotor which form part of the induction passage to the working chambers of the engine such that air plus fuel is drawn through and thereby cools the rotor. A disadvantage of this system is that the air/fuel mixture becomes heated during its passage through the rotor and hence the volumetric efficiency and the power output of the engine is impaired.
Partially to overcome this disadvantage, alternative systems are known where only the induction air is drawn through the rotor, the air then passing through a plenum chamber and/or an intercooler before the fuel is added and the mixture then inducted into the working chamber. Nevertheless, the power is lower than in oil cooled rotor types of engine because of the pressure loss associated with the complex induction system, and the inability to cool the air down to ambient levels with a practical size of intercooler.
Rotary piston engines of this type are set forth in Pat. Nos. GB 1385687 and GB 1386811.
A further known type of rotary piston engine with air cooled rotor uses means to force ambient air through the rotor and then eject it direct to atmosphere and not be inducted by the engine, the air that is inducted to the working chamber being a separate supply of air at ambient pressure and temperature. Alternative known means of forcing the air through the rotor passages include use of an engine-driven centrifugal fan, or use of an exhaust ejector, or use of ram air only, such ram air resulting from the velocity of the machine or vehicle in which the engine is mounted through the atmosphere.
A rotary piston engine of this type is set forth in Pat. No. EP 0273653.
By these means these types of engines will give a higher power output than the oil cooled rotor types of engine as a result of the higher mechanical efficiency of the air cooled rotor type combined with the same level of volumetric efficiency as the oil cooled rotor type.
A disadvantage of the type of engine that ejects the spent rotor cooling air to atmosphere is that the oil which lubricates the bearings and rubbing surfaces inside the rotor is used for only one pass through the rotor and is then inevitably ejected with the cooling air. In some applications of the engine, such as the powering of unmanned air vehicles, this emission of wet oil particles may be acceptable, but in most ground and marine application, this is unacceptable. Another disadvantage is that the oil consumption is relatively high because separate oil supplies have to be provided for both the rotor internals and the inner epitrochoidal peripheral surface upon which the apex seals are sliding.
To counteract these problems, in another type of rotary piston engine, ejected rotor cooling air plus oil is passed through an oil separator before ejection and so a proportion of oil is captured and available for re-use. It is difficult though to make the oil separator 100% efficient in collecting the oil without a pressure loss to the cooling air passing through the oil separator being high, thereby requiring the engine-driven rotor cooling fan to require a high power input. For this reason it is not practical to combine the use of an efficient oil separator with the ram air or the exhaust ejector systems of forcing the air through the rotor.
In all rotary engines with air cooled rotors described above, the cooling of the rotor is such that the systems require careful design to avoid the rotor overheating in adverse conditions of operation.
There exist potential applications of these engines where known methods of cooling of the rotor by air are not suitable and the oil cooled rotor system has to be employed. For example, the use of a rotary piston engine with an air cooled rotor would not be appropriate with turbocharging or other means of forced induction when the heat input to the rotor is considerably increased relative to a naturally aspirated engine. This situation is particularly true for the important application of turbocharging of engines fitted to aircraft which are required to fly with good performance at higher altitudes. The purpose and effect of turbocharging is to maintain high power in a less dense atmosphere which results in continuing high level of heat input to the rotor which coincides with a reduced ability of the less dense air to cool the rotor.